This invention relates generally to in-process balancing of a rotating mass by the creation of an electric arc across an air gap which erodes material from the mass. More specifically the invention involves a new and improved means to initiate and maintain the electric arc discharges which erode the material.
In recent years advances in electronic vibration measurement equipment have made possible accurate determination of the amount and location of minute unbalance in rotating elements such as gyroscope rotors and small turbine wheels. Such rotating elements are often required to be balanced to levels approaching the limits of the measuring equipment. But while it is possible to electronically measure the quantity of material creating unbalance, no means has yet been devised for physically removing the exact minute amounts indicated. Consequently, trial and error correction is employed. An estimated amount of stock is removed manually by milling, grinding, or by electrochemical methods. Unbalance is then measured to determine the effect of the correction and how much unbalance remains. This two-step process of alternate correction and measurement continues until an acceptable state of balance is attained.
Ideally, correction should occur simultaneously with measurement in a single operation. By continuously monitoring the effect of gradual stock removal lower levels of unbalance can be achieved in less time.
Rotating elements not requiring critical balancing would also benefit from such an ideal in-process balancing system in cases where controlled stock removal methods such as drilling or welding metal strip are not suitable. Even when controlled correction methods are utilized, in-process balancing may function for trim balancing to a precision not otherwise attainable except by the trial and error method previously described.
The present invention in general relates to a new and improved unbalance correction system wherein correction is attained by electric arc discharge through a heavy spot on a rotating mass to remove material from the heavy spot by vaporization.
The concept of starting an arc with a superimposed high voltage has long been utilized in automatic arc welding equipment. As applied in the balancing field the principle is found in prior art U.S. Pat. No. 2,322,561 which indicates use of high voltage to initiate a low voltage capacitor discharge across an air gap for stock removal purposes.
One aspect of the present invention involves in-process correction of unbalance by means of periodic electrical discharges occurring between stationary electrodes and a workpiece rotating in a balancing machine of conventional design. Heat generated by the high-current arc so formed melts and vaporizes stock from the heavy spot of the workpiece. Each discharge results in an incremental reduction of unbalance and proceeds automatically to a minimal level of unbalance. The residual unbalance corresponds at most to the amount of material removed per discharge. Unbalance will never exceed this minimal level because the location of unbalance will change as soon as over-correction occurs. Thus it is impossible to over-shoot, and the system is inherently self-limiting once the minimal level is reached.
In one embodiment, two electrodes are positioned in close proximity to the workpiece and to each other. The electrodes are designated positive and negative and are connected via a heavy cable to positive and negative terminals respectively of a capacitor bank. An electrical circuit is completed from the positive to the negative electrode by imposition of the workpiece which functions as an intermediate passive conductor. This circuit comprises the workpiece and the air gap which exists on opposite sides of the workpiece, the electrodes being spaced from the workpiece.
Discharge of the capacitor bank occurs in response to a signal from the balancing machine's electronic vibration measurement instrument. This signal is synchronous with vibration from the unbalanced workpiece and is phase adjustable to time the discharge when the heavy spot is immediately adjacent the electrodes.
Initial breakdown of the gap typically requires over 10,000 volts. The capacitors of the capacitor bank however are typically charged to only several hundred volts. Once breakdown of the gap has been initiated, only about 50 volts is required across the gap to sustain the high current flow. Breakdown is initiated by superimposing a high breakdown voltage upon the low-voltage/high-current main arc circuit to establish an ionized path across the gap. The invention provides a means for developing and applying such a high voltage to the gap to initiate breakdown and allow for the subsequent discharge of the capacitors to sustain and augment the electric arc. Once started the discharge is self-sustaining until the capacitor bank charge voltage is lowered to the gap-maintaining potential of about 50 volts.
The invention, in its broader aspects, contemplates both single and multiple capacitor banks. Where only a single capacitor bank is used, a repeat cycle timer can permit arc discharges to occur at spaced time intervals, rather than once per revolution, to allow re-charging time for the capacitor bank.
A more specific aspect of the invention relates to the use of multiple capacitor banks to improve the operating efficiency.
All capacitors have an ESR (Equivalent Series Resistance) rating. This value, expressed in ohms, is typically so small as to be negligible under ordinary conditions. ESR is the result of capacitor dielectric losses (hysteresis) developed in an alternating electric field. Since these losses are manifest as heat, capacitors subject to large ripple currents undergo a significant temperature rise. The heat generated in watts is equal to (I.sub.R).sup.2 (ESR) where I.sub.R is the ripple current. At some temperature the dielectric begins to break down, accelerating losses and heat buildup until the capacitor is permanently damaged.
In a capacitor discharge system extreme fluctuations in voltage occurring during each charge/discharge cycle represent a detrimental "ripple current". Ripple current is proportional to charge voltage, capacitance and discharge frequency. If voltage and/or capacitance are increased, the repetition rate (discharge frequency) must be decreased to avoid capacitor damage. Consequently the rate of unbalance correction is determined by ripple current heating limitations.
One obvious solution would be to sequentially switch a plurality of capacitor banks, each bank operating within the restrictions stated above. However, conventional switch gear capable of switching the high currents involved (20,000 amps) would be so large as to render the system impractical.
One feature of this invention is to provide a means for sequentially switching two or more banks of capacitors without employing large switch gear usually associated with currents of this magnitude.
In brief, a plurality of capacitor banks are connected in parallel via "gap switches" with the unbalance correction electrode circuit. Rate of unbalance correction is thereby increased proportional to the number of capacitor banks. For example, a 4-bank system can have an overall repetition rate four times that of a single bank system.
As an ancillary benefit the gap switches isolate the charged capacitors from the electrode circuit so that the electrode area where loading and unloading of parts takes place is not exposed to electrified electrodes connected directly to charged capacitors.
The so-called "gap switches" comprise a specific feature of this invention. They permit switching very high currents without moving parts such as found in conventional switches. The gap switch functions as a "contactless contact" to turn on a current. Once turned on current will continue to flow until the gap switch voltage falls below about 30 volts. Without means for turn-off, except as noted above, the gap switch is useful only in A.C. or periodic discharge circuits. In functional terms the gap switch is similar to an SCR (Silicon Controlled Rectifier). SCR's, however, cannot be utilized due to the destructive high voltages required to trigger capacitor discharge.
The disclosed construction of the gap switches represents further attributes of the invention, as will be seen from the ensuing detailed description of the preferred embodiment.
The foregoing features, advantages and benefits of the invention, in its several aspects, along with additional ones, will be seen in the ensuing description and claims which should be considered in conjunction with the accompanying drawings. The drawings disclose a preferred embodiment of the invention according to the best mode contemplated at the present time in carrying out the invention.